As a control method of a vehicle power plant, a control method using a state space model is known. Japanese Unexamined Patent Application Publication No. 2002-187464, for example, discloses a technology for calculating various manipulation quantities of an engine and a continuous variable transmission by a model tracking control method. In the model tracking control method disclosed in the gazette, an acceleration command value indicating preferable responsiveness of an acceleration estimated value corresponding to target acceleration is outputted from a standard model, and a deviation between the acceleration command value and the acceleration estimated value is integrated by an integrator. Then, an acceleration deviation integrated quantity and each state quantity of the standard model and vehicle model to be controlled are fed back and each manipulation quantity is calculated. Moreover, the gazette also describes a gain switching method in such acceleration feedback control. According to the switching method, an initial value of the integrator is calculated so that each manipulation quantity of the engine and the continuous variable transmission before and after the gain switching, that is, an input variable in a state formula of a plant model continuously changes.
However, the switching method disclosed in the gazette has a problem. In the switching method disclosed in the gazette, though continuity of the input variable is kept, continuity of a state variable relating to a behavior of the power plant is not guaranteed. Thus, it is likely that the behavior of the power plant is disturbed before and after the switching. Moreover, in the control of the vehicle power plant, there might be a case in which a plurality of controllers used in manipulation of an actuator are prepared, and the controller to be used is switched in accordance with an operation condition or an operation state, but the problem in the switching method disclosed in the above gazette may occur also when the controller operating the actuator is switched to another controller.